This invention relates to a method and apparatus to permit the pyrolysis of an agglomerative or a caking coal without plugging of the pyrolysis or reactor chamber by solid products of the pyrolysis reaction.
The use of fluidized systems wherein a fluidized stream is formed of finely divided coal particles, heated char particles and a carrier stream to pyrolyze the coal particles to extract the volatiles therefrom is well known in the art. In such prior art processes the heated char particles and/or the gas stream are utilized to provide the requisite heat of pyrolysis to the coal particles with a supply of char continuously being produced upon pyrolysis of the coal in the system.
Agglomerative particulate bituminous coals are well known to those skilled in the art due to the tendency of such coals to plasticize and become sticky at relatively low temperatures, e.g. 550.degree. to 850.degree. F. When such prior art processes have been applied to agglomerative bituminous coal, problems have arisen due to the agglomerative nature of such coal. Thus, when the coal particles are heated as noted above to their plastic state, and the particles in such condition make contact with the walls of the reactor, the particles cake thereon to form a bubbly compact mass which swells and then resolidifies, forming a solid coherent body with a porous structure, that is, coke. Such agglomeration of coal particles on the reactor walls causes severe blockage in the system and renders the system inoperable.
In an effort to overcome the blockage or plugging problems encountered in pyrolysis systems utilizing agglomerative coals, various procedures have been suggested by the prior art. Thus, in U.S. Pat. Nos. 2,955,077 and 3,375,175 the agglomerative particulate coal is preheated in a conventional fluidized bed at temperatures ranging from 600.degree. to 825.degree. F. for periods ranging from 1 to 30 minutes to remove at least a portion of the volatiles from the coal so that the coal can be further pyrolyzed to recover the volatiles therefrom. The requirement of preheating agglomerative bituminous coals in these processes for relatively long residence times imposes severe economic limitations on these processes.
U.S. Pat. No. 3,357,896 discloses heating large particles of caking coal through their plastic range in a free fall system to avoid contact with the reactor walls and produce non-caking coal char. The patent also discloses the use of oxygen in the heating gas to prevent caking of the coal while it is heated through its plastic range. However, such treatment with oxygen has the disadvantage that it substantially reduces the yield of hydrocarbons produced during pyrolysis. It is desirable to obtain as product of the pyrolysis reaction a maximum yield of liquid or gaseous hydrocarbons.
In Squires U.S. Pat. Nos. 3,597,327 and 3,855,070, coal is introduced into a fluidized bed of coke pellets of relatively large size ranging from about 1/12" to about 1/2" in diameter. Such coal particles are heated almost instantly to the reaction temperature and become sticky, and are captured by and accrete upon the pellets, forming a smear upon the surface of the pellets.
Nelson U.S. Pat. No. 2,582,711 discloses introduction of coal and char into the bottom of a draft tube of a recirculating bed of char. The coal is conveyed upwardly in the draft tube and induces the upward flow of char in the draft tube and the internal recirculation of the char diluent. Heat to the system is supplied by means of hot fuel gas.
Other patents illustrating prior art pyrolysis and carbonization processes and systems include Phinney, U.S. Pat. No. 2,709,675 and Landers, U.S. Pat. No. 3,455,789.
However, to applicant's knowledge, many of the prior art processes and systems have been unsatisfactory and relatively few of such prior art processes and systems have been practiced commercially.
In my above copending application, Ser. No. 633,898, filed Nov. 20, 1975, now U.S. Pat. No. 4,163,693, there is disclosed a mixing apparatus for mixing particulate materials, such as a turbulent jet stream of coal and a fluidized char stream and passing the resulting turbulent mixture stream of char and coal through a reactor to produce gaseous products. The coal stream is directed downwardly into the upper end of a mixing section which communicates with a reactor, and entrains and mixes with heated recycled char fed from an annular fluidizing chamber. It is stated therein that those coals which exhibit a plastic or tacky state will lose their tackiness upon being heated sufficiently. Thus, by heating the primary entraining stream of coal particles sufficiently during contact with the secondary or entrained hot char recycle stream in such jet mixer apparatus, sufficient heat is transferred to the coal particles to get such materials through the tacky state rapidly enough after mixing to avoid the plugging problem.
It is an object of the invention to provide an efficient economical and continuous method and apparatus for pyrolyzing agglomerative coals in a pyrolysis chamber or reactor for recovery of volatile hydrocarbons under conditions which prevent plugging of the reactor. A particular object is the provision of a process and apparatus for rapidly heating agglomerative coal during pyrolysis under conditions to permit passage of the coal through the tacky or plastic state prior to impingement of the pyrolyzed solid residue on the reactor walls, thus avoiding formation of coke deposits on the reactor walls and formation of a plug within the reactor, while also avoiding thermal decomposition of the gaseous hydrocarbon products and obtaining high yields thereof. Yet another particular object is to provide a process of the above type and an apparatus, utilizing therein as a heat source a fluidized bed of char surrounding a jet of coal particles, without appreciable recirculation of the char entrained in the coal jet within the pyrolysis chamber.